Aircraft engine control systems employ a number of feedback sensors to accomplish closed loop control of engine actuators. For linear displacement engine components, such as a nozzle actuator, a linear displacement sensor is needed to generate a feedback signal so that engine electronic control (EEC) systems can reliably determine the actuator position.
Known EEC systems have incorporated a variety of analog or digital displacement sensors. Two analog sensors are the linear variable differential transducer (LVDT), and the linear variable phase transducer (LVPT). A LVDT displacement sensor produces an output signal whose output voltage magnitude varies as a function of displacement. In contrast, a LVPT displacement transducer generates an output signal whose phase, rather than magnitude, is modulated as a function of displacement. An example of a linear variable phase transducer is disclosed in U.S. Pat. No. 4,134,065.
Since the output signal of an LVPT sensor is not amplitude based, it possesses several potential advantages over an LVDT sensor in a control system, including greater accuracy over varying temperatures, and it requires simpler, less costly electronics for implementation.
An LVPT is characterized by primary windings or coils which are energized by alternating current sources having 90.degree. of phase separation. Encompassed by the primary coils or windings is a core which moves with the actuator stroke. The magnetic flux produced by the primary coils is summed in the displaceable core in proportion to the cores position with respect to the primary coils, producing an output voltage across a secondary coil whose phase is shifted with respect to a primary coil excitation signal.
Some existing systems which employ a LVPT have a coarse capability of monitoring the transducer for feedback sensor failure. Since the voltage amplitude of the LVPT is approximately constant, a substantial change in the voltage across the secondary coil can be sensed as an indication of LVPT failure. However, it has been determined that the voltage amplitude across the secondary coil is not sufficiently constant to provide an indication of transducer failure for high precision applications, such as a displacement sensor within an aircraft engine actuator. Moreover, existing displacement feedback fault detection systems cannot discriminate between various types of faults. These LVPT faults or failures include loss of LVPT excitation signals, a primary or secondary coil open circuit, or full or partial primary or secondary short circuit. For high precision applications in aircraft engines, it is critical that the control system quickly and accurately determine if there has been a displacement feedback sensor failure.